1. Field of the Invention
The present invention relates to a probe unit for a scanning type of tunnel current detecting device for, for example, a scanning tunnel microscope and a high-density recording-reproducing apparatus utilizing the principle of the microscope.
The present invention relates also to a method for driving a probe suitable for the probe unit.
The present invention further relates to a scanning-type tunnel current detecting device provided with the probe unit.
2. Related Background Art
A scanning tunnel microscope (hereinafter referred to as STM) is already known which utilizes the tunnel effect in which electric current flows through a barrier between the surface of a specimen and a sharp electroconductive probe brought as close as several nm or less to the surface. [See G. Binnig et al., Helvetica Physica Acta, 55, 726 (1982); U.S. Pat. No. 4,343,993, etc.]
The tunnel current which flows on application of a voltage between a probe and the surface of a specimen brought to close proximity of several nm or less will change exponentially as a function of the distance therebetween. Accordingly, the surface of the specimen can be observed at high resolution in an atomic level by scanning the surface of the specimen in X and Y directions in matrix with the probe with the tunnel current kept constant.
A high-density recording-reproducing apparatus which is based on the principle of the STM is described in Japanese Patent Application Laid-Open Nos. 63-161552 and 63-161553. In such an apparatus, recording is conducted, with a probe similar to that of STM, by changing the voltage applied between the probe and a recording medium which exhibits memory effect in its switching characteristics. The material for the recording medium is exemplified by a thin layer of a chalcogen compound or of an organic compound having a .pi.-electron system. The reproduction is conducted according to the difference in tunnel resistance between the recorded portion and the nonrecorded portion of the thin layer. In this type of recording, recording-reproducing is practicable by use of a recording medium the surface shape of which can be changed by application of a voltage to the probe.
Japanese Patent Application Laid-Open No. 61-206148 (1986) describes a probe formed by utilizing a processing technique [K. E. Peterson "Silicon as a Mechanical Material", Proceedings of the IEEE, 70 (5), 420-457 (1982)] for providing a minute structure on a substrate, and also describes an STM based on such a technique. This probe unit is constructed of a silicon single crystal as a substrate, a parallel spring made by fine fabrication and being finely movable in X-Y directions, and a tongue-shaped member having a probe provided on the movable portion so that the probe may be displaced in the direction perpendicular to the substrate face (in the Z direction) by electrostatic force caused by an applied electric field between the tongue-shaped part and the bottom face portion.
Japanese Patent Application Laid-Open No. 62-281138 describes a memory device provided with a converter array in which tongue-shaped parts as disclosed in Japanese Patent Application Laid-Open No. 61-206148 are arranged in a multiple manner.
In conventional probe units of a cantilever structure, there are posed, for example, the following disadvantages (1) to (4).
(1) The probe, since it is provided on the tip end of the cantilever, tends to positionally deviate slightly by thermal expansion or contraction of the cantilever in the length direction, or by deformation caused by difference in thermal expansion coefficient between the cantilever material and the electrode material.
(2) A cantilever constituting a probe is difficult to from precisely because of liability of causing warpage or torsion by the internal stress in preparation thereof. The cantilever is also liable to be deformed by internal stress relaxation with a lapse of time. Accordingly, a cantilever is unsatisfactory for a probe-driving mechanism which is required to have position-controllability with precision of an atomic level. A multiple arrangement of cantilevers, for example, as shown in the above-mentioned Japanese Patent Application Laid-Open No. 62-281138, required to maintain the mutual positional relation with high precision. The cantilever, however, could not satisfy such requirement.
(3) In the aforementioned prior art, the substrate is prepared by finely processing a silicon single crystal, and the material for the substrate is limited to silicon, which needs many steps of production processing, resulting in a disadvantageously high cost.
(4) Further in the prior art, the driving mechanism has a plurality of probes on parallel hinges. Therefore, on driving each of the probes in the z direction, each probe may be deviated by electrostatic force, causing mutual interference of the probes, if the parallel hinges are not sufficiently rigid.